Scanning device including an objective lens formed of two materials

ABSTRACT

An optical scanning device ( 1 ) for scanning an information layer ( 4 ) of an optical record carrier ( 2 ), the information layer ( 4 ) being covered by a transparent layer ( 3 ) of thickness t d  and refractive index n d . The device comprises a radiation source ( 11 ) for generating a radiation beam ( 12, 15, 20 ) and an objective system ( 18 ) for converging the radiation beam on the information layer. The objective system is characterised in comprising a lens comprising a synthetic resin on a substrate, the total thickness t of the lens satisfying the condition: Formula (I), where FWD+t d /n d &lt;0.51, and FWD is the free working distance between the lens ( 18 ) and carrier ( 2 ) and Φ is the entrance pupil diameter of the lens ( 18 ), where t, t d , Φ and FWD are expressed in millimeters.

FIELD OF THE INVENTION

The present invention relates to an optical scanning device for scanningan optical record carrier, to a lens system suitable for, but notlimited to, use as an objective lens in such a scanning device, and tomethods for manufacturing such a device and such a system.

BACKGROUND TO THE INVENTION

In optical recording, increasingly the trend is towards miniaturisationof both the optical record carriers and the devices used to scan (e.g.write to and/or read from) the carriers. Examples of optical recordcarriers include CDs (compact discs) and DVDs (digital versatile discs).

In order for the optical record carriers to be made smaller, without adecrease in information storage capacity, the information density on thecarrier must be increased. Such an increase in information density mustbe accompanied by a smaller radiation spot for scanning the information.Such a smaller spot can be realised by increasing the numerical aperture(NA) of the objective system used for focusing a radiation beam in thescanning device on the record carrier. Consequently, it is desirable tohave a lens with a high numerical aperture (e.g. NA=0.85).

Conventional high NA objective lenses consist of two elements in orderto ease the manufacturing tolerances, at the expense of introducing anextra assembly step to align the elements making up the objective lens.

The Japanese article “Single Objective Lens Having Numerical Aperture0.85 for a High Density Optical Disk System” by M Itonga, F Ito, KMatsuzaki, S Chaen, K Oishi, T Ueno and A Nishizawa, Jpn. J. Appl. Phys.Vol. 41. (2002) pp. 1798–1803 Part 1, No. 3B March 2002, describes asingle objective lens, having two aspherical surfaces, with a relativelyhigh NA of 0.85. The lens is made of glass. The lens diameter is 4.5 mm,and the lens has an aperture diameter of 3.886 mm. This single elementlens does not require the extra alignment assembly step needed by thetwo-element objective lens. Because of the high value of NA, theobjective lens becomes more susceptible to variations in themanufacturing process i.e. manufacturing tolerances. Therefore, forthese high NA objective lenses the manufacturing tolerances play an evenmore important role in the designing process than was the case forobjective lenses having a lower numerical aperture.

In order for scanning devices to decrease in size, it is desirable thatthe components within the scanning devices (such as the objective lens)are made as small as possible.

However, it is not possible to simply scale down large lens designs toproduce smaller lenses, as the lens design is dependent upon theproperties of the optical recording medium. For instance, the lensdesign is dependent upon the properties of the transparent layer thattypically covers the information layer on an optical record carrier, andwhich the scanning radiation beam must traverse. In the scaling downprocess the thickness of the cover layer of the disc remains unaffected(the same record carrier is likely to be used for both the normal sizedobjective lens and the small sized objective lens). Hence, the design ofa small sized objective lens suitable for scanning the optical recordmedium will be substantially different from the design of a normal sizedobjective lens.

Further, whilst it is desirable that the objective lens is formed of asingle element (assembling two small elements is difficult and thereforerather expensive), forming a single element solely out of glass isrelatively expensive. The glass moulding production process requireshigh temperatures to melt the glass, and relatively large forces toshape the melted glass, thus making the resulting lens a relativelyexpensive component.

A cheaper alternative method of manufacturing a single element lens isto form a synthetic resin on a flat or spherical substrate (such asglass). For instance, glass spheres are relatively cheap to manufacture,and so truncated glass spheres are ideal substrates. Synthetic resinsmay be applied to the surface of the substrate so as to provide thedesired (e.g. aspherical) surface shape. U.S. Pat. No. 4,623,496describes how such a liquid synthetic resin can be applied to asubstrate, with the synthetic resin being subsequently cured so as toform a layer having a predetermined desired aspherical curvedcharacteristic.

It will be appreciated that design constraints for lenses formed using asynthetic resin on a substrate will differ from design constraints forlens formed from a single substance such as glass. For instance, thesynthetic resin will typically have a different refractive index thanthe substrate.

It will also be appreciated that as lenses are made smaller, high NAlenses remain susceptible to variations in the manufacturing processi.e. manufacturing tolerances.

FIG. 1A shows an example of an objective lens 18, having a glass body200 with a substantially spherical surface 181, and a substantially flatsurface 182. Such a glass body would subsequently have at least onelayer of a synthetic resin applied to the first surface 181 so as toform an aspherical surface. It will be appreciated that if the glassbody is formed or aligned incorrectly, then the performance of the lensformed with the addition of the resin will be impacted. The lens is oftotal thickness t along the optical axis (i.e. thickness of body plusresin layer(s)).

In the examples shown in FIGS. 1A–1D, two separate layers of resin 100,102 are applied to respective surfaces 181, 182 of the glass body 200.Each layer of resin 181, 182 is shaped so as to form a respectiveaspherical surface.

Subsequent FIGS. 1B, 1C and 1D respectively illustrate how the substrateshape and orientation can vary due to variations in thickness, decentreand tilt of the two aspherical surfaces relative to the desired opticalaxis 19 (in each instance, the original position of the surface 181 isillustrated by a dotted line).

FIG. 1B illustrates the overall thickness of the lens being greater thanthe desired thickness, in this instance due to the spacing in betweenthe surfaces 181, 182 being larger than desired. However, it will beappreciated that the two surfaces could in fact be spaced closertogether than desired as well.

FIG. 1C illustrates decentre of the two aspherical surfaces. In thisexample, the glass body 200 has been located shifted in a directionperpendicular to the ideal position relative to the desired optical axis19, with the centre of the aspherical surface 100 being off the desiredoptical axis 19, whilst the aspherical surface 102 remains centred onthe axis 19.

FIG. 1D illustrates how the glass body, including surface 181, is tiltedi.e. rotated in relation to the desired rotationally symmetric positionalong the principal axis, resulting in a tilt of the upper asphericalsurface 100, relative to the lower aspherical surface 102.

It is an aim of embodiments of the present invention to provide anobjective lens formed from a synthetic resin on a substrate materialcapable of withstanding reasonable manufacturing tolerances.

In optical scanning devices, radiation beams may enter the objectivelens obliquely, due to inaccurate alignment of the objective lens withinthe scanning device, variations in the position of the recording carrierrelative to the scanning device, or due to radiation beams beingutilised that do not travel along the optical axis. For instance, suchoff-axis beams are typically used to provide information on positioningof the scanning radiation spot on the record carrier.

Such oblique beam entrance results in wave-front aberrations. Typicallyan allowance in the root mean square of the optical path difference(OPD_(rms)) of approximately 0.07λ (where λ is the wavelength of therelevant radiation beam), in total is allowed for wave-front aberrationsof the scanning beam for the total optical scanning device, such thatthe system is diffraction limited. It can be convenient to express theOPD_(rms) in mλ (where 0.001λ=1 mλ). The field of the lens system is thearea within which oblique beams generate an OPD_(rms) of less than 15mλ. The field of view of the lens system is twice the field.

It is an aim of embodiments of the present invention to provide an smallsized high NA objective lens formed of a synthetic resin on a substratethat is tolerant to oblique beam entrance to the lens and tolerant formanufacturing errors.

SUMMARY OF THE INVENTION

In a first aspect, the present invention provides an optical scanningdevice for scanning an information layer of an optical record carrier,the information layer being covered by a transparent layer of thicknesst_(d) and refractive index n_(d), the device comprising a radiationsource for generating a radiation beam and an objective system forconverging the radiation beam on the information layer, the objectivesystem being characterised in comprising a lens comprising a syntheticresin on a substrate, the total thickness t of the lens satisfying thecondition:

$0.8 < \frac{t - {1.1\phi} + 1.1}{1.18 - {2.28\left\lbrack {{FWD} + \frac{t_{d}}{n_{d}}} \right\rbrack}} < 1.2$where FWD+t_(d)/n_(d)<0.51, and FWD is the free working distance betweenthe lens and carrier and φ is the entrance pupil diameter of the lens,where t, t_(d), φ and FWD are expressed in millimetres.

By designing a lens to satisfy such constraints, the resulting lens istolerant to oblique beam entrance and manufacturing errors.

In another aspect, the present invention provides a lens systemcomprising at least one lens for converging a radiation beam on aninformation layer of an optical record carrier, the information layerbeing covered by a transparent layer of thickness t_(d) and refractiveindex n_(d), the lens system being characterised in comprising a lenscomprising a synthetic resin on a substrate, the total thickness t ofthe lens satisfying the condition:

$0.8 < \frac{t - {1.1\phi} + 1.1}{1.18 - {2.28\left\lbrack {{FWD} + \frac{t_{d}}{n_{d}}} \right\rbrack}} < 1.2$where FWD+t_(d)/n_(d)<0.51, and FWD is the free working distance betweenthe lens and carrier and φ is the entrance pupil diameter of the lens,where t, t_(d), φ and FWD are expressed in millimetres.

In a further aspect, the present invention provides a method formanufacturing a lens system comprising at least one lens formed of asynthetic resin on a substrate, for converging a radiation beam on aninformation layer of an optical record carrier, the information layerbeing covered by a transparent layer of thickness t_(d) and refractiveindex n_(d), the method comprising the steps of: forming the substrate,the total thickness t of the lens satisfying the condition:

$0.8 < \frac{t - {1.1\phi} + 1.1}{1.18 - {2.28\left\lbrack {{FWD} + \frac{t_{d}}{n_{d}}} \right\rbrack}} < 1.2$where FWD+t_(d)/n_(d)<0.51, and FWD is the free working distance betweenthe lens and carrier and φ is the entrance pupil diameter of the lens,where t, t_(d), φ and FWD are expressed in millimetres.

In another aspect, the present invention provides a method ofmanufacturing an optical scanning device for scanning an informationlayer of an optical record carrier, the information layer being coveredby a transparent layer of thickness t_(d) and refractive index n_(d),the method comprising the steps of: providing a radiation source forgenerating a radiation beam; providing a lens system for converging theradiation beam on the information layer, the lens system beingcharacterised in comprising a lens comprising a synthetic resin on asubstrate, the total thickness t of the lens satisfying the condition:

$0.8 < \frac{t - {1.1\phi} + 1.1}{1.18 - {2.28\left\lbrack {{FWD} + \frac{t_{d}}{n_{d}}} \right\rbrack}} < 1.2$where FWD+t_(d)/n_(d)<0.51, and FWD is the free working distance betweenthe lens and carrier and φ is the entrance pupil diameter of the lens,where t, t_(d), φ and FWD are expressed in millimetres.

Other aspects of the invention will be apparent from the dependentclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention, and to show how embodimentsof the same may be carried into effect, reference will now be made, byway of example, to the accompanying diagrammatic drawings in which:

FIGS. 1A, 1B, 1C and 1D show a lens having a substantially sphericalfirst surface, and a substantially flat surface with the sphericalsurface being respectively: in the desired position, too far away fromthe second surface, decentred with respect to the second surface, andtilted with respect to the second surface;

FIG. 2 shows a device for scanning an optical record carrier includingan objective system;

FIG. 3 shows two alternative types of lens designs utilised byembodiments of the present invention;

FIG. 4 illustrates, as an average of different objective lens designs,the wave-front abberation arising from respectively field, thickness,decentre and tilt as a function of refractive index, as well as the rootmean square total wave-front abberation arising from these four factors;

FIG. 5 illustrates, as an average of different objective lens designs,the wave-front abberation arising from respectively field, thickness,decentre and tilt as a function of normalised power of the surface ofthe objective lens facing the record carrier, as well as the root meansquare total wave-front abberation arising from these four factors; and

FIG. 6 illustrates the average optimum thickness of the objective lensand the optimum refractive index n for the substrate of the objectivelens for differing objective lens designs, as a function of the FreeWorking Distance (FWD).

DETAILED DESCRIPTION OF THE EMBODIMENTS

FIG. 2 shows a device 1 for scanning an optical record carrier 2,including an objective system 18 according to an embodiment of thepresent invention. The record carrier comprises a transparent layer 3,on one side of which an information layer 4 is arranged. The side of theinformation layer facing away from the transparent layer is protectedfrom environmental influences by a protection layer 5. The side of thetransparent layer facing the device is called the entrance face 6. Thetransparent layer 3 acts as a substrate for the record carrier byproviding mechanical support for the information layer.

Alternatively, the transparent layer may have the sole function ofprotecting the information layer, while the mechanical support isprovided by a layer on the other side of the information layer, forinstance by the protection layer 5 or by a further information layer anda transparent layer connected to the information layer 4. Informationmay be stored in the information layer 4 of the record carrier in theform of optically detectable marks arranged in substantially parallel,concentric or spiral tracks, not indicated in the Figure. The marks maybe in any optically readable form, e.g. in the form of pits, or areaswith a reflection coefficient or a direction of magnetisation differentfrom their surroundings, or a combination of these forms.

The scanning device 1 comprises a radiation source 11 that can emit aradiation beam 12. The radiation source may be a semiconductor laser. Abeam splitter 13 reflects the diverging radiation beam 12 towards acollimator lens 14, which converts the diverging beam 12 into acollimated beam 15. The collimated beam 15 is incident on an objectivesystem 18.

The objective system may comprise one or more lenses and/or a grating.The objective system 18 has an optical axis 19. The objective system 18changes the beam 15 to a converging beam 20, incident on the entranceface 6 of the record carrier 2. The objective system has a sphericalaberration correction adapted for passage of the radiation beam throughthe thickness of the transparent layer 3. The converging beam 20 forms aspot 21 on the information layer 4. Radiation reflected by theinformation layer 4 forms a diverging beam 22, transformed into asubstantially collimated beam 23 by the objective system 18 andsubsequently into a converging beam 24 by the collimator lens 14. Thebeam splitter 13 separates the forward and reflected beams bytransmitting at least part of the converging beam 24 towards a detectionsystem 25. The detection system captures the radiation and converts itinto electrical output signals 26. A signal processor 27 converts theseoutput signals to various other signals.

One of the signals is an information signal 28, the value of whichrepresents information read from the information layer 4. Theinformation signal is processed by an information processing unit forerror correction 29. Other signals from the signal processor 27 are thefocus error signal and radial error signal 30. The focus error signalrepresents the axial difference in height between the spot 21 and theinformation layer 4. The radial error signal represents the distance inthe plane of the information layer 4 between the spot 21 and the centreof a track in the information layer to be followed by the spot.

The focus error signal and the radial error signal are fed into a servocircuit 31, which converts these signals to servo control signals, 32for controlling a focus actuator and a radial actuator respectively. Theactuators are not shown in the Figure. The focus actuator controls theposition of the objective system 18 in the focus direction 33, therebycontrolling the actual position of the spot 21 such that it coincidessubstantially with the plane of the information layer 4. The radialactuator controls the position of the objective system 18 in a radialdirection 34, thereby controlling the radial position of the spot 21such that it coincides substantially with the central line of track tobe followed in the information layer 4. The tracks in the Figure run ina direction perpendicular to the plane of the Figure.

The device of FIG. 2 may be adapted to scan also a second type of recordcarrier having a thicker transparent layer than the record carrier 2.The device may use the radiation beam 12 or a radiation beam having adifferent wavelength for scanning the record carrier of the second type.The NA of this radiation beam may be adapted to the type of recordcarrier. The spherical aberration compensation of the objective systemmust be adapted accordingly.

In order to provide a single element high-NA (NA>0.65) objective lensfor use in an objective system as described above, we propose that thelens is made using a thin aspherical correction layer of a syntheticresin on surface of a substrate (such as glass), in similar manner asproposed in U.S. Pat. No. 4,623,496. This layer is sometimes referred toas a replica layer. In order to make the glass body cheap tomanufacture, it is preferable that the body has the shape of a truncatedglass sphere. Suitable synthetic resins can be formed from monomeric oroligomeric acrylates, with the synthetic resin being subsequently curedunder UV light. Diacryl is one example of such a resin. Diacryl has arefractive index of 1.5987 at a wavelength of 405 nm and Abbe number34.5.

FIG. 3 illustrates two alternative lens designs (labelled respectivelyType I and Type II). In both instances, the lenses are indicatedpositioned in relation to a respective optical record carrier 2, havingan information layer 4 and a transparent covering layer 3. In bothcases, an incoming radiation beam 15 is being converged 20 onto theinformation layer 4, through the covering transparent layer 3. It willbe seen that the radiation beam 15, 20 is at an oblique angle ofincidence to the optical axis 19 of the lens 18.

Both type I and type II lenses comprise a glass body 200 in the shape ofa truncated glass sphere. Such a shape can be formed by forming a glasssphere, and then cleaving the glass sphere. Such a cleaved surface isrelatively flat.

Type I lenses are characterised in only having a single asphericalsurface. This surface is formed by applying the synthetic resin 100 tothe curved surface of the truncated glass sphere substrate. In use, thisaspherical surface of the lens is facing the radiation source.

Type II lenses are characterised in having two aspherical surfaces. Atype II lens can be envisaged as a type I lens, with an additionalaspherical surface formed on the flat side of the glass body (i.e. thesurface of the glass body which, in use, is adjacent to the recordingmedium 2). Such a second aspherical surface is formed by a further layer102 of synthetic resin.

Below will now be described in more detail the various preferred designconstraints of type I and type II lenses, followed by a tablesummarising the parameters of three example lenses. In this table,example lens 2 corresponds to a type I lens, and example lenses 1 and 3to type II lenses. Note that a type I lens can be considered as aspecial case of type II i.e. the case where the second asphericalsurface is flat.

TYPE I

Type I lenses are more cost effective than type II lenses, as theyrequire a synthetic layer 100 to be added to only one surface of theglass body 200.

Preferably, the total thickness t of the lens (the thickness along theoptical axis 19) complies with the relationship:

$\begin{matrix}{0.8 < \frac{t - {1.1\phi} + 1.1}{1.18 - {2.28\left\lbrack {{FWD} + \frac{t_{d}}{n_{d}}} \right\rbrack}} < 1.2} & (1)\end{matrix}$where FWD+t_(d)/n_(d)<0.51 and where φ is the entrance pupil diameter ofthe objective lens. It is assumed that the relevant thickness anddistance dimensions (t, FWD, φ and t_(d)) are measured in millimetres.

Even more preferably, the thickness t of the lens complies with:

$\begin{matrix}{0.9 < \frac{t - {1.1\phi} + 1.1}{1.18 - {2.28\left\lbrack {{FWD} + \frac{t_{d}}{n_{d}}} \right\rbrack}} < 1.1} & (2)\end{matrix}$

The free working distance corresponds to the distance the lens can bemoved before contacting the record carrier i.e. the distance from thesurface of the lens facing the record carrier to the surface of thetransparent layer 3, as measured along the optical axis.

Preferably, the refractive index n of the glass body of the lenscomplies with:

$\begin{matrix}{{- 0.05} < {n - 2.49 + {2.79\left( \frac{{FWD} + \frac{t_{d}}{n_{d}}}{F} \right)} - {2.28\left( \frac{{FWD} + \frac{t_{d}}{n_{d}}}{F} \right)^{2}}} < 0.05} & (3)\end{matrix}$where FWD is the free working distance, t_(d) the thickness of thetransparent layer 3, n_(d) the refractive index of the transparent layer3, and where F is the focal length of the lens.

In use in a scanning device, the lens might be used in combination withdifferent wavelengths of radiation (different wavelengths may be used toread and write data). Alternatively the wavelength of the radiationsource (e.g. a laser) may change as a function of the power of theradiation beam (different powers may be utilised to read and write datato an information recording medium). In order that the lens is tolerantto such variations in wavelength, it is preferable that the Abbe numberof the glass body is greater that 40.

TYPE II

This design consists of two aspherical surfaces. It is preferable thatthe second aspherical surface (the aspherical surface formed on the flatside of the glass body) is substantially flat. Preferably, the absolutevalue of the best fit radius R of the surface complies with:

$\begin{matrix}{{\text{}R\text{}} > {5\frac{n_{r} - 1}{N\; A}\phi}} & (4)\end{matrix}$where φ is the entrance pupil diameter of the lens, NA is the numericalaperture and n_(r) the refractive index of the resin. The best fitradius R is the radius of a sphere which has minimal root mean squaredeviation from the aspherical surface.

Similarly, it is preferable that the normalised paraxial optical power P(the optical power of the surface divided by the total optical power ofthe lens system) of the second aspherical surface of the lens complieswith the relationship:−0.1<P<0.1  (5)

Preferably, the total thickness t of the lens complies with relation(1).

Even more preferably, the total thickness t complies with relation (2).

Preferably, the refractive index n of the glass body of the lenscomplies with relation (3).

Again, in order that the lens is sufficiently tolerant to vacations inwavelength of the radiation, it is preferable that the Abbe number ofthe glass body is greater than 40.

Table 1 provides details of three explicit lens designs optimised inorder to be tolerant for field, thickness variation of the substrate,decentre of the aspheric surfaces and tilt between the aspheric surfacesaccording to embodiments of the present invention. Example 2 is of typeI (and thus a special case of type II) while example 1 and 3 are of typeII. The performance of the various designs, including examples 1 and 3,is indicated within FIGS. 4, 5 and 6, which were used to derive theabove preferred design relationships. The performance of the explicitdesign example 2 is tabulated in table 2.

TABLE 1 Example 1 2 3 Numerical aperture 0.85 0.85 0.85 Entrance pupildiameter (mm) 1.0 1.0 1.0 Wavelength (nm) 405 405 405 Glass body (glasstype) N-LAK14 S-LAM60 LASFN9 Refractive Index glass body 1.7180 1.76891.8983 Abbe number glass body 55.4 49.3 32.17 Body radius 0.5175 0.53020.5570 Body thickness (mm) 0.6411 0.6697 0.6321 Refractive index resin1.5987 1.5987 1.5987 Abbe number resin 34.5 34.5 34.5 Free workingdistance FWD 0.15 0.15 0.15 (mm) 1st asphere Replica thickness center(μm) 20 18.8 15.8 B2 (mm⁻²) 1.154981 1.1282466 1.058627 B4 (mm⁻⁴)0.778878 0.7171389 0.554093 B6 (mm⁻⁶) 0.436929 0.2726619 0.13553 B8(mm⁻⁸) 2.201988 1.9733778 0.34716 B10 (mm⁻¹⁰) −8.35349 −9.2510925−6.98564 B12 (mm⁻¹²) −22.3366 −17.943089 −10.4856 2nd asphere Replicathickness center (μm) 30 0 30 B2 (mm⁻²) −0.12437 0 0.28866 B4 (mm⁻⁴)1.163883 0 −1.59621 B6 (mm⁻⁶) −12.6129 0 5.803061 B8 (mm⁻⁸) 89.0167 021.4475 B10 (mm⁻¹⁰) −272.507 0 −232.761 B12 (mm⁻¹²) 0 0 0 Best fitradius (mm) −6.2407 infinity 2.2992 Normalised power 2^(nd) asphere 0.090 −0.20

TABLE 2 Root mean square wavefront aberration WFA Performance of example2 (ml) 0.1° field 8.1 1 μm thickness difference lens body 9.8 0.01° tiltbetween asphere and flat exit surface 8.8 10 μm decenter between asphereand flat exit surface 0

The front and back surfaces of the example lenses each have a rotationalsymmetric aspherical shape which is given by the equation:

${z(r)} = {\sum\limits_{i = 1}^{6}{B_{2i}r^{2i}}}$with z the position of the surface in the direction of the optical axisin millimetres, r the distance to the optical axis in millimetres, andB_(k) the coefficient of the k-th power of r. The values of B_(k) forthree different example lens designs are listed in Table 1, in which the1^(st) asphere is assumed to be the surface of the lens facing theradiation source. To calculate the normalised optical power P of thesecond aspherical surface the following formula is usedP=B ₂(1−n _(r))φ/NAwhere B₂ is the first aspherical coefficient of the second surface,n_(r) the refractive index of the resin, φ is the entrance pupildiameter of the objective lens and NA the numerical aperture.

Examples 1, 2 and 3 fulfil the requirements of equations (1) and (2).Furthermore, example 2 fulfils the requirement of equation (3). Examples1 and 2 fulfil both the requirements of equations (4) and (5). Finally,the Abbe number of the glass bodies of examples 1 and 2 is larger than40.

FIG. 4 shows the root mean square of the wave-front aberrations due toan oblique radiation beam entrance to the lens of 0.1° field, with theglass substrate body of the lens having a 1 μm thickness difference (thedeviation of the actual body from the desired thickness t), 10 μmdecenter aspheres and 0.01° tilt aspheres for various designs, includingexamples 1 and 3 of the objective lens designs tabulated in table 1, asa function of the refractive index of the objective lens. The individualroot mean square of the wave-front aberration (WFA) arising due to eachof the contributions from the field, thickness, decenter and tilt isindicated, as is the root mean square (RMS) total wave-front aberration.Note that the performance of lens design example 2 (see table 2) is inthe with FIG. 4, except for decentre of the asphere which is exactlyzero for example 2 because of the flat exit surface.

Similarly, FIG. 5 shows the root mean square of the wave-frontaberration due to 0.1° field, 1 μm thickness difference, 10 μm decenteraspheres and 0.01° tilt aspheres for the same objective lens designs, asa function of normalised power of the second surface of the objectivelens (the lens surface facing the recording medium).

In both FIGS. 4 and 5, the lenses have a numerical aperture=0.85, and anentrance pupil diameter of 1.0 mm. The aspherical surface(s) is (are)formed by a layer of Diacryl upon a glass body. The glass body is shapedas a truncated glass sphere. The radiation beam has a wavelength λ=405nm, and a free working distance (FWD) of 0.15 mm was utilised, with arecording medium having a covering layer (transparent layer 3) thicknessof 0.1 mm, of refractive index 1.6223.

FIG. 6 shows the optimum thickness of the objective lens and the optimumrefractive index of the glass body for various objective lens designsincluding example 1 and 3 of the designs tabulated in table 1 as afunction of the free working distance (FWD). This data assumes that eachof the lenses has a NA=0.85, and an entrance pupil diameter 1.0 mm. Aradiation beam of wave length λ=405 nm was utilised, in conjunction witha recording medium (e.g. a disk) having a covering layer thickness of0.1 mm and a refractive index of 1.6223.

FIG. 4 shows that as the refractive index n increases, the field andtilt tolerance increase (i.e. the WFA decreases), whilst the thicknesstolerance decreases. The decenter tolerance shows an optimum (i.e.minimum) near n=1.78. Combining all four tolerances shown in the figure,it is shown that the optimum lens designs are found when therelationship shown in equation 3 holds.

FIG. 5 shows that, with increasing normalised power of the secondsurface, the disc field and tilt tolerance decreases, while thethickness tolerance increases. The decenter tolerance shows an optimumnear a normalised power (P) of −0.025.

In both FIGS. 4 and 5, the minimum of the decenter curve does notcorrespond to zero WFA as is the case for example 2 of table 1. Thismeans that the second asphere in the case of type II designs near zeropower is not flat, due to at least one of the coefficients B₄–B₁₂ beingnon-zero.

FIG. 6 shows that the optimum total thickness of the objective lensalong the optical axis (i.e. the total thickness of the glass body andthe resin layers) and the optimum refractive index n of the glass bodyof the objective lens both decrease for increasing free working distance(FWD). Providing fits to the points illustrated in FIG. 6, shows thatthe optimum refractive index (n_(opt)) is given byn _(opt)=2.21794–3.9321*FWD+6.60614*FWD²and the optimum thickness as a function of FWD is given byt=1.03616–2.27542*FWD

In both cases FWD and t are expressed in millimetres.

From these specific results and FIGS. 4, 5 and 6, taking into accountscaling relationships, it has been possible to deduce the optimum lensdesign parameters shown in equations 1, 2, 3 and 5.

Finally, when equation (4) holds the resin thickness to be applied onthe substantially flat side of the glass body remains small. As aresult, the effect of any shrinkage of this layer during manufacturingremains small too, which eases the manufacturing of the objective lens.

It will be appreciated that different embodiments of the invention canbe applied in relation to a variety of lens systems. Preferably,embodiments are utilised in respect of lens systems that have anumerical aperture of greater than 0.7. Preferably, lens systems inaccordance with embodiments have an entrance pupil diameter of less than2 mm, and even more preferably, less than 1.5 mm. Preferably,embodiments are utilised in conjunction with radiation beams having awavelength of less than 600 nm, including beams having wavelengths ofapproximately 405 nm.

Whilst the above embodiments have been described in conjunction withlenses formed only of the resin diacryl on a glass substrate, it will beappreciated that the parameters of the present invention are appropriatefor lens designs formed of any synthetic resin on any transparentsubstrate. Suitable classes of materials for the resin are aromatic andaliphatic di-(meth-)acrylates, aromatic and aliphatic bis-epoxides,bis-oxetanes, bis-vinylethers. More specificaly a bisphenol A baseddimethacrylate (“diacryl 101”, also known as2,2-Bis(4-methacryloxyphenyl)propane) can be used.

In view of the above examples, it will be appreciated that embodimentsof the invention can be used to provide objective lenses formed from asynthetic resin on a substrate capable of withstanding reasonablemanufacturing tolerances. Further, embodiments of the present inventionare tolerant to oblique beam entrance to the lens.

1. An optical scanning device for scanning an information layer of an optical record carrier, the information layer being covered by a transparent layer of thickness t_(d) and refractive index n_(d), the device comprising a radiation source for generating a radiation beam and an objective system for converging the radiation beam on the information layer, the objective system being characterised in comprising a lens comprising a synthetic resin on a substrate, the total thickness t of the lens satisfying the condition: $0.8 < \frac{t - {1.1\phi} + 1.1}{1.18 - {2.28\left\lbrack {{FWD} + \frac{t_{d}}{n_{d}}} \right\rbrack}} < 1.2$ where FWD+t_(d)/n_(d)<0.51, and FWD is the free working distance between the lens and carrier and φ is the entrance pupil diameter of the lens, where t, t_(d), φ and FWD are expressed in millimetres.
 2. A device as claimed in claim 1, wherein the total thickness t of the lens satisfies the condition: $0.9 < \frac{t - {1.1\phi} + 1.1}{1.18 - {2.28\left\lbrack {{FWD} + \frac{t_{d}}{n_{d}}} \right\rbrack}} < 1.1$
 3. A device as claimed in claim 1, wherein the refractive index n of the substrate satisfies the condition: ${- 0.05} < {n - 2.49 + {2.79\left( \frac{{FWD} + \frac{t_{d}}{n_{d}}}{F} \right)} - {2.28\left( \frac{{FWD} + \frac{t_{d}}{n_{d}}}{F} \right)^{2}}} < 0.05$ where F is the focal length of the lens.
 4. A device as claimed in claim 1, wherein the Abbe number of the substrate is greater than
 40. 5. A device as claimed in claim 1, wherein the surface of the lens arranged to face the record carrier has a best fit radius satisfying the condition: $R > {5\;\frac{n_{r} - 1}{NA}\phi}$ where φ is the entrance pupil diameter of the lens, NA is the numerical aperture of the lens, and n_(r) is the refractive index of the resin.
 6. A device as claimed in claim 1, wherein the normalised optical power P of the surface of the lens arranged to face the record carrier satisfies the condition: −0.1<P<0.1
 7. A lens system comprising at least one lens for converging a radiation beam on an information layer of an optical record carrier, the information layer being covered by a transparent layer of thickness t_(d) and refractive index n_(d), the lens system being characterised in comprising a lens comprising a synthetic resin on a substrate, the total thickness t of the lens satisfying the condition: $0.8 < \frac{t - {1.1\phi} + 1.1}{1.18 - {2.28\left\lbrack {{FWD} + \frac{t_{d}}{n_{d}}} \right\rbrack}} < 1.2$ where FWD+t_(d)/n_(d)<0.51, and FWD is the free working distance between the lens and carrier and φ is the entrance pupil diameter of the lens, where t, t_(d), φ and FWD are expressed in millimetres.
 8. A lens system as claimed in claim 7, wherein said substrate is glass.
 9. A method for manufacturing a lens system comprising at least one lens formed of a synthetic resin on a substrate, for converging a radiation beam on an information layer of an optical record carrier, the information layer being covered by a transparent layer of thickness t_(d) and refractive index n_(d), the method comprising the steps of: forming the lens, the total thickness t of the lens satisfying the condition: $0.8 < \frac{t - {1.1\phi} + 1.1}{1.18 - {2.28\left\lbrack {{FWD} + \frac{t_{d}}{n_{d}}} \right\rbrack}} < 1.2$ where FWD+t_(d)/n_(d)<0.51, and FWD is the free working distance between the lens and carrier and φ is the entrance pupil diameter of the lens, where t, t_(d), φ and FWD are expressed in millimetres.
 10. A method as claimed in claim 9, further comprising the step of forming an aspherical surface on said substrate by applying a synthetic resin to a surface of said substrate.
 11. A method of manufacturing an optical scanning device for scanning an information layer of an optical record carrier, the information layer being covered by a transparent layer of thickness t_(d) and refractive index n_(d), the method comprising the steps of: providing a radiation source for generating a radiation beam; providing a lens system for converging the radiation beam on the information layer, the lens system being characterised in comprising a lens comprising a synthetic resin on a substrate, the total thickness t of the lens satisfying the condition: $0.8 < \frac{t - {1.1\phi} + 1.1}{1.18 - {2.28\left\lbrack {{FWD} + \frac{t_{d}}{n_{d}}} \right\rbrack}} < 1.2$ where FWD+t_(d)/n_(d)<0.51, and FWD is the free working distance between the lens and carrier and φ is the entrance pupil diameter of the lens, where t, t_(d), φ and FWD are expressed in millimetres. 